Since the 1970s, the optical fiber communication technology has developed rapidly for lots of advantages such as its ultra-large transmission bandwidth and ultra-low transmission loss. The optical fiber sensing technology has also developed rapidly as the optical fiber communication technology is developing. The optical fiber sensing technology refers to a technology that uses light waves as carriers and optical fibers as media to measure external physical signals (such as temperature and strain) or various parameters of optical fibers. Compared to the traditional mechanical or electromagnetic sensors, an optical fiber sensor has enormous advantages, such as: it is not affected by the noise caused by electromagnetic interference; it can operate stability under a strong electromagnetic environment; it can be operated in hazardous locations for example, flammable and explosive places, since the optical fiber is an electrical insulator that does not generate electric sparks; it may have an excellent compatibility with an optical fiber communication system to realize ultra-long-range sensing; and the like.
The optical reflectometer technology as an important member in a family of optical fiber sensing technologies refers to a technology for non-destructive detection of an optical fiber network using optical fiber back-scattered light and can be used to measure the distribution conditions of fiber length, loss, connectors, fracture and the like. One of the most important optical reflectometer technologies at present is optical time domain reflectometer (OTDR) technology. The OTDR technology has the advantages like long detection distance up to hundreds of kilometers; simple system structure; low cost; and the commercial products are currently available on the market. The spatial resolution of the OTDR technology (the minimum distance between two adjacent “event points” can be distinguished) depends on the width of the optical pulse, and the narrower the optical pulse is, the higher the spatial resolution is. However, the optical pulse cannot be made to be quite narrow under the limitations of the performances of a laser device and the non-linear effect of the optical fiber. Therefore, the spatial resolution of the OTDR technology is poor, which limits the application of the OTDR technology.
In order to solve the problem of spatial resolution, the researchers proposed an optical frequency domain reflectometer (OFDR) technology. The spatial resolution of the OFDR technology depends on a frequency tuning range of an optical source. As long as the frequency tuning range of the optical source is larger, the theoretical spatial resolution is higher. However, the OFDR technology also faces two major problems. First, the detection distance of the OFDR technology is relatively short, and the maximum detection distance generally does not exceed half of a coherence distance of a laser device. It has been reported in a literature that an auxiliary interferometer is used for phase noise compensation to increase the detection distance [Opt. Lett. 32(22), 3227-3229 (2007)], but this technology has a high hardware complexity and long data processing time due to complex phase noise compensation algorithm, and it cannot compensate phase noise introduced by environmental factors. Secondly, the frequency tuning range of the optical source is limited, and therefore, the spatial resolution is hardly increased. It has been reported in a literature that a narrow-linewidth laser device is modulated using a radio frequency sweep signal source and a single-sideband modulator to obtain a wide-range linear frequency-swept optical source, thereby achieving high spatial resolution [J. Lightwave Technol. 6, 3287-3294 (2008)]. This scheme has now become the mainstream choice for the externally modulated OFDR system. However, the single-sideband modulator has the disadvantages of complicated use, high cost, large insertion loss, etc., and more seriously it cannot completely suppress the other sidebands to realize single-sideband frequency sweeping, which badly affects the frequency sweeping performance. In addition, the sweeping range of this scheme is limited by the performances of a radio frequency sweep signal source. Therefore, it is very necessary to find an optical reflectometer that achieves high spatial resolution and long detection distance.
It was found by searching the prior art that Chinese Patent Document No. CN103763022A (published on Apr. 30, 2014) disclosed a high spatial resolution optical frequency domain reflectometer system based on high-order sideband frequency sweeping modulation, which comprises a frequency-swept optical source part, a test optical path part, a receiver and a signal processing part, wherein: the frequency-swept optical source part uses a narrow-linewidth laser device as an original optical source; and emergent light generates a frequency-swept sideband optical signal by means of external modulation. During the external modulation: the radio frequency sweep signal is amplified by a high-power radio frequency amplifier and then applied to an electro-optic modulator with a relatively low half-wave voltage in a high-voltage mode, so that multi-order sidebands are generated and filtered through a narrow-band optical filter to generate a high-order broadband frequency-swept optical sideband; the high-order optical sideband serves as a frequency-swept carrier wave optical source to be guided into an optical path system; backscattered and reflected optical signals are collected; local coherent detection and signal processing are performed; and thereby an optical frequency domain reflection analysis is implemented. However, the hardware complexity of this technology is high, and the filtering effect is limited by the performances of the filter, such that the other sidebands cannot be completely suppressed, which seriously affects the frequency sweeping performance; after the rest sidebands are filtered out, the optical power loss is extremely high, and a high-magnification optical amplifier is required for amplification to bring extra phase noise.